Tyre changing machines are commonly used for fitting/removing tyres of vehicle wheels onto/from respective rims.
As is known, before being able to completely remove a tyre, it is necessary to release its beads from the seat of the respective rim.
To perform this job, a bead breaking arm is used mounted on the tyre changing machine itself.
In practice, the known tyre changing machines include a bead breaking unit comprising a bead breaking arm arranged horizontally and having a first extremity hinged to a side of the body or base of the machine itself, outside of it.
The arm has a second extremity having a bead breaking tool, also called “blade”.
The arm is operated by a pneumatic or hydraulic cylinder which is also associated with the side of the base on which the bead breaking arm is hinged, but on the inner side of the body.
On said side of the body, the base has a protruding portion provided with an abutting surface that cooperates with the blade to carry out bead breaking.
The pneumatic or hydraulic cylinder is operated through one or more pedal controls positioned in the front part of the base, where, however, they are particularly uncomfortable to use taking into account the fact that, during bead breaking, the operator also needs to keep a hand on the blade or on a grip handle of the blade itself.
To overcome this drawback bead breaking units are known, such as that shown in the patent document U.S. Pat. No. 7,343,955, in which the pneumatic or hydraulic cylinder is operated through a control lever, substantially horizontal, which is mounted on an upright that rises from the bead breaking arm.
The control lever operates a valve that opens and closes a pneumatic circuit for feeding the cylinder, for its movement in one direction rather than the other.
Even this embodiment, however, has several drawbacks.
It should be noticed, in fact, that during bead breaking, particularly in case of very hard tyres, the release of the tyre from the rim takes place suddenly when the force applied by the blade exceeds a certain threshold value, beyond which the bead is separated from the rim.
At the time of release, the bead stops resisting the advancement of the blade almost instantaneously and the bead breaking arm, without opposition, rotates abruptly and also drags with it the operator's hand holding the control lever.
The bead breaking job is therefore rather uncomfortable and unpleasant for the operator, who is pulled and suffers the backlash of the bead breaking arm that rotates almost instantaneously.
Furthermore, at the time of sudden release of the bead, most of the times the operator loses control sensitivity of the control lever, with the risk of not being able to properly control the stopping of the bead breaking arm after releasing, and therefore of damaging the tyre or the wheel rim.